A. Diffusive Screens
In many applications there is the need for devices whose purpose is to spread an illumination beam over a certain field of interest with a desired intensity variation. Such devices are generally referred to in the art as diffusive screens, diffusion plates, or diffusers.
In its simplest version, a diffusive screen is made of a rough surface with a relief pattern that can be typically described by Gaussian statistics. To fabricate such diffusive screens several methods have been proposed. Among these one can distinguish three basic categories.
First, there are diffusive screens based on a random surface structure (ground glass). Such diffusive screens are commercially available at low cost. However, because there is little control over their diffusing characteristics, the performance of such screens is very limited and only of interest in applications with very flexible and loose requirements.
A second class of diffusive screens is obtained by holographic recording of a speckle pattern. This class offers more flexibility than ground glass screens in tailoring the diffusion pattern. However, such holographic diffusive screens tend to generate images with a grainy appearance, which may be unpleasant for viewing purposes. Also, the sudden intensity variations associated with speckles lead to non-uniform illumination over restricted viewing angles.
The third class of diffusive screens include those where a certain substrate has its surface modified according to some relief pattern. An example includes arrays of microlenses which provide light diffusion. This third class offers better control of the relief pattern than either ground glass screens or holographic screens.
There has been considerable effort to address the problem of light diffusion as briefly summarized by the following U.S. patents.
U.S. Pat. No. 4,427,265 discloses a diffusive screen with an irregular arrangement of curved surfaces superposed on a periodic microlens array. The goal is to maintain the light diffusing properties while avoiding some of the artifacts associated with the underlying periodic array. The curvature of each microlens is controlled on average.
U.S. Pat. No. 5,733,710 describes various arrangements of microlenses generated by mask exposure with microlens location being varied through mask rotation. It also discloses the combination of a diffusive screen structure and a Fresnel lens on opposite sides of the same substrate.
U.S. Pat. No. 4,826,292 discloses a diffusion plate with a relief structure composed of cones created by ion bombardment and etching.
U.S. Pat. No. 5,871,653 discloses fabrication methods to obtain a diffusive screen structure based on microlens arrays for use in flat panel displays.
Some of the issues that must be addressed when designing a diffusive screen include controllable viewing angles, controllable intensity variation over the useful viewing field, resolution, absence of visual artifacts, and efficient use of the incident illumination. To achieve full control of design capabilities and obtain the best possible diffusing performance for a given application one must be able to control the surface-relief pattern with adequate precision.
The relief control achieved in the prior art is limited to simple arrangements where individual structures might have some curvature or optical power. In particular, existing art in the fabrication of microlens arrays includes, among others, the techniques disclosed in U.S. Pat. Nos. 5,871,653, 5,536,455, 5,324,623, and 5,300,263. Current methods are based on polymer melting, thermal relaxation, ion exchange diffusion, surface tension effects, and etch smoothing. These methods offer little control over the microlens shape, except that it is nearly spherical.
The quality obtained in the prior art is a largely statistical process because there has been no strict control of the positioning and/or shaping of the structures used to achieve diffusion. The elementary structures that compose the arrays are conventionally nearly spherical shapes. As often found in the patent literature, the elementary structures that compose a diffusive screen are loosely described as “curved” simply because there is little control over their shape. For other types of relief structures not described by curved microlenses, the surface is obtained by random means of only statistical control, such as surface bombardment.
It is therefore clear that there exists a need for well-controlled diffusing surfaces with elementary microstructures that are well defined and chosen to meet specific diffusion requirements.
B. Display Screens
Applications that involve the display of information require appropriate means of delivery to allow the user some form of interaction with the information, be it access to a database or simply watching a movie. Such systems are usually composed of (1) a light engine which provides illumination, (2) optics to transfer the optical information, and (3) a display screen which provides the immediate delivery of the visual information to the user. The light engine and optics are, for all practical purposes, invisible to the user.
The display screen, however, represents the element of direct contact with the user and, for this reason, needs to embody in the best possible way the performance of the system. In other words, the display screen provides the immediate impression to the user and the quality of the image it can provide determines, to a great extent, the acceptance or not of a particular system.
Some of the issues relevant to the performance of display screens are efficiency (brightness), resolution (ability to resolve features and avoid aliasing effects), gain (scattering over specified angular range), low speckle (graininess of image associated with random structures of some screen surface designs), contrast (clear distinction between colors), and ambient light rejection (screen looks black when light engine is turned off). These are just some of the issues that must be taken into account in the design of the light engine, optics, and display screen, since these all work together.
Traditionally, the approaches used to design display screens have been the same as those used for diffusing screens. Thus, display screens have incorporated random elements in the screen surface without, however, being able to closely control the shape of the micro-scatterers or the scattering pattern. The simplest screens have been in the form of classical ground glass diffusers. Other devices have included holographic diffusers and microlens arrays. In most of these cases, some element of randomness has been introduced by the recording of a speckle pattern or by superposing and distributing microlens shapes in a random fashion.
Thus, as with diffusing screens, there exists a need in the art for well-controlled display screens with elementary microstructures that are well defined and chosen to meet specific diffusion requirements.